This invention relates to a process for the production of pellets of metal powder, more particularly highly active metal powder, which may be used for example as fixed-bed catalysts in chemical processes. Fixed-bed catalysts of the type in question are used in catalytic gas-phase and trickle-phase processes in which the reactants are passed through or over a catalyst bed. This is described, for example, in DE-A 3 403 973.
A fixed-bed catalyst normally has well-defined geometric shapes, for example rings or cylinders. The cylindrical shape is the most commonly encountered. Considerable importance is attributed both to the crushing strength of the individual shaped pellets of the catalyst bed, because inadequate compressive strength leads to disintegration of the pellets and hence to contamination of the reaction product, and to a correspondingly large inner surface of the individual pellets. The pellets are normally produced under a defined high pressure in a pelleting machine using catalyst powder as the starting material.
If fine-particle, more particularly powder-form, metals, for example those of group VIII of the periodic system, are used as catalyst materials, the pelleting process has to be carried out in an oxygen-free atmosphere to avoid surface oxidation which among other things is because of the large surface of the highly active powders. Such oxidation is highly exothermic and hence involves a considerable risk of fire and possibly explosions.
The metal powders are normally supplied for processing in the form of an aqueous suspension or slurry. Production of the metallic pellets then comprises the following steps:
drying the metal powder in an inert gas atmosphere,
pelleting the powder in a pelleting machine, again in an inert gas atmosphere, and
storing the pellets produced in water or another liquid.